The present invention relates to a method and apparatus for applying coating material on a moving web of paper, and in particular to a coating method and apparatus of the trailing blade type wherein light, medium or heavy weight coatings may be applied in a novel and improved manner.
Conventional coaters of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible coater or doctor blade located on the trailing side of the applicator, which serves to doctor or level the applied coating. In general, an excess of coating material is applied on the web and the coater blade then meters or removes the excess while uniformly spreading the coating onto the web surface.
In recent years it has become desirable to produce papers having a minimum amount of coating. To achieve low coat weights with conventional trailing blade equipment it is necessary to increase the pressure of the coater blade against the web, which results in a high rate of wear of the blade and necessitates more frequent blade replacement. High blade pressure also increases the possibility of web breaks and streaking caused by foreign particles caught between the blade and web.
Conventional coaters employ a relatively long dwell or soak time, which is the time interval between initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive web coats without intervening drying, because the successive coats tend to migrate into and contaminate the previous coat.
The foregoing problems are discussed in U.S. Pat. No. 3,348,526 issued to Neubauer, wherein a narrow stream of coating is extruded onto an inverted trailing blade that defines a nip region with a supported web. The coating application is such that the coating material is unpressurized after leaving an orifice and being supported on the blade, and the leading side of the coating material stream is exposed to the environs in the zone of application. Since the coating is bladed substantially immediately after application, soak times are kept to a minimum.
To overcome the disadvantages of the aforementioned applicators in applying lightweight coatings on paper, there has been developed a short dwell time applicator as disclosed in U.S. Pat. No. 4,250,211 issued to Damrau et al and assigned to the assignee of the present invention. In that applicator, coating material is introduced in excess into a relatively narrow application zone for being applied on a web carried therethrough. A forward wall of the applicator defines a relatively narrow gap with the web at the upstream end of the application zone, and excess material in the zone overflows through the gap and forms therein a liquid seal, so that coating material in the zone and as applied to the web is maintained under pressure. The speed of the web is adjusted for a relatively short dwell time, and a flexible coater blade doctors the web at the downstream end of the zone, thereby removing excess material from the web and at the same time uniformly spreading the material on the web. In consequence of the short dwell time of the pressurized application of coating material on the web, an appropriate yet lightweight amount of coating may be applied without need for high blade pressures.
A difficulty occasionally encountered in use of conventional applicators as well as the type disclosed in U.S. Pat. No. 4,250,211 is that, depending upon the nature of the paper and the coating material and the amount of material to be applied on the web, the coating may fail to properly cover the surface of the web. This results in "skipping" in the applied coating, which impairs its quality.